The number of deaths around the world from cancer each year continues to be of major concern, with only a few treatments being available for specific types of cancer, and these having no absolute guarantee of success. Most treatments rely on a general "shotgun" approach, killing off rapidly growing cells in the hope that rapidly growing cancerous cells will succumb, either to the treatment, or at least be reduced in numbers to allow the body's system to eliminate the remainder.
The search for cures has been hampered by the discovery that different forms of cancer require different treatments. Given that virtually any part of the body can be affected by cancer, the task becomes enormous.
Nevertheless, despite their differences, cancers also share a number of similarities. Prime amongst these is the growth of undifferentiated tissue. However, even this is not 100% accurate, in that certain cancerous cells do exhibit a degree of differentiation, and this is shown in the sex cancers, such as those of breast and testicle, where tumors may be positive or negative for hormone receptors. Treatment of these tumors depends on the hormone state, and may be as simple as administration of the relevant hormone antagonist, such as tamoxifen.
Another factor which most cancers share is that, in order to be fatal, they must metastasize. Until such time as metastasis occurs, a tumor, although it may be malignant, is confined to one area of the body. This may cause discomfort and/or pain, or even lead to more serious symptoms, but if it can be located, it may be surgically removed and, if done with adequate care, cause no further problems.
However, once metastasis sets in, surgical resection may remove the parent tumor, but cancerous cells have invaded the body, and only chemotherapy, or some particular form of targeting therapy, stands any chance of success.
Thus, the ability to invade locally and to metastasize in organs distant from the primary tumor (tumor progression) is the lethal event in the course of most cancers. Alteration/degradation of the extracellular matrix (ECM) surrounding the primary tumor, and modifications of the tumor cell adhesive properties, are known to be crucial for dissociation of the metastatic cells from the primary tumor cells (Liotta, Cancer Res. 46:1-7 (1986); Hart et al., Biochim. Biophys. Acta 989:65-84 (1989)).
Tumor angiogenesis is essential for both primary tumor expansion and metastatic tumor spread, and angiogenesis itself requires ECM degradation (Blood et al., Biochim. Biophys. Acta 1032:89-118 (1990)). Thus, malignancy is a systemic disease in which interactions between the neoplastic cells and their environment play a crucial role during evolution of the pathological process (Fidler, I. J., Cancer Metastasis Rev. 5:29-49 (1986)).
Identifying the alterations in gene expression which are associated with malignant tumors, including those involved in tumor progression, is clearly a prerequisite not only for a full understanding of cancer, but also to develop new rational therapies against cancer. Mutations and/or abnormal control of expression of two groups of cellular genes (the proto-oncogenes and the tumor suppressor genes) have been shown to lead in a multistep process to the loss of normal growth control and to the acquisition of the transformed cell phenotype (Weinberg, R. A., Cancer Res. 49:3713-3721 (1989)). However, the molecular mechanisms which lead to tumor progression are much less clear (Nowell, P. C., Cancer Res. 46:2203-2207 (1986); Fidler, I. J., Cytometry 10:673-680 (1989)).
Thus, a further problem arises, in that the genes characteristic of cancerous cells are very often host genes being abnormally expressed. It is quite often the case that a particular protein marker for a given cancer is over-expressed in that cancer, but is also expressed elsewhere throughout the body, albeit at reduced levels.
Some of the proteins associated with cancers are enzymes which break down the extracellular matrix, which is important for maintaining cells in their proper relationship to each other. One such class is the metalloproteinases (MMPs) (Matrisian, L. M., Trends Genet. 6:121-125 (1990)), so called because they bind zinc. However, none has been found to be diagnostic, of cancer, or any particular tumors, although the presence of some may be indicative.
MMPs are involved in a number of physiological and pathological processes in which ECM remodelling and cell migration are implicated, e.g. morphogenesis and embryonic development, rheumatoid arthritis, and tumor invasion and metastasis. MMP inhibitors are known to be able to block tumor invasion and angiogenesis, which are crucial for tumor progression, in experimental models.
All members of the matrix metalloproteinase family are proteinases which degrade at least one component of ECM, are secreted in a latent form and require activation, such as proteolysis (e.g. by plasmin) to become active. Interstitial collagenases specifically attack connective tissue collagens (I to III), whereas type IV collagenases (72 kD and 92 kD) degrade collagens present in the basement membrane and fibronectin. Stromelysins (transins)-1 and -2, and also pump-1, have a much broader substrate specificity, degrading proteoglycans, laminin, fibronectin, and collagens (III to V).
In man, most of the malignant tumors are carcinomas, and among non-smokers, breast cancer is the leading cause of mortality by cancer in woman (Willett, W., Nature 338:389-394 (1989)). The expression of several oncogenes has been reported to be altered in malignant breast cells and tumors, but no particular pattern of oncogene/suppressor gene expression can be consistently associated with breast cancer (Gullick, W. J., Prog. Growth Factor Res. 2:1-13 (1990)).
However, the neoplastic cells of breast tumors are often embedded in an adipose and mesenchymal stroma, which may also be important in control of their proliferation and in their ability to metastasize. Indeed, it is known that stroma cells can modulate, both positively and negatively, the growth of normal mammary epithelium (Salomon et al., in Breast Cancer: Cellular and Molecular Biology (eds., Lippman, M. E. and Dickson, R. B.), pp. 363-389 (Kluwer, Boston, (1988)), and that interactions between the epithelial and stromal components can influence epithelial carcinogenesis in the mammary gland (DeOme et al., Cancer Res. 38:2103-2111 (1978)).
The existence of "activated" (Tremblay, G. Exp. Mol. Pathol. 31:248-260 (1979)) and/or abnormal (Grey et al., Proc. Nat. Acad. Sci. USA 86:2438-2442 (1989)) fibroblasts in malignant breast tumors has been postulated, and it has been proposed that breast cancer could represent a partial escape from dependence on a stromal requirement or an abnormally strong response to a stromal component.
Owing to the nature of cancerous tissue, it is usually relatively easy to set up a continuous culture, or cell line, of a given cancer, a process which makes it easy to study the effects of a given treatment regimen. A significant drawback to such systems lies in their very nature--while the test treatment will establish whether it can act directly against the cells, it is by no means certain what effect the treatment will have in vivo, and biochemical analysis of such lines is inevitably in the absence of the tissue normally surrounding the tumor in vivo.